With the advent of inexpensive digital color printers, methods and systems of color digital halftoning have become increasingly important in the reproduction of printed or displayed images possessing continuous color tones. It is well understood that most digital color printers operate in a binary mode, i.e. for each color separation, a corresponding color spot is either printed or not printed at a specified location or pixel. Digital halftoning controls the printing of color spots, where the spatial averaging of the printed color spots by either a human visual system or a viewing instrument, provide the illusion of the required continuous color tones.
The most common halftone technique is screening, which compares the required continuous color tone level of each pixel for each color separation with one or more predetermined threshold levels. The predetermined threshold levels are typically defined for a rectangular cell that is tiled to fill the plane of an image, thereby forming a halftone screen of threshold values. At a given pixel, if the required color tone level is darker than the given halftone threshold level, a color spot is printed at that specified pixel. Otherwise the color spot is not printed. The output of the screening process is a binary pattern of multiple small “dots,” which are regularly spaced as is determined by the size, shape, and tiling of the halftone cell. In other words, the screening output, as a two-dimensionally repeated pattern, possesses two fundamental spatial frequencies, which are completely defined by the geometry of the halftone screen.
It is understood in the art that the distribution of printed pixels depends on the design of the halftone screen. For clustered-dot halftone screens, all printed pixels formed using a single halftone cell typically group into one or more clusters. If a halftone cell only generates a single cluster, it is referred to as a single-dot halftone or single-dot halftone screen. Alternatively, halftone screens may be dual-dot, tri-dot, quad-dot, or the like.
While halftoning is often described in terms of halftone dots, it should be appreciated by those skilled in the art that idealized halftone dots can possess a variety of shapes that include rectangles, squares, lines, circles, ellipses, “plus signs,” X-shapes, pinwheels, and pincushions, and actual printed dots can possess distortions and fragmentation of those idealized shapes introduced by digitization and the physical printing process. Various digital halftone screens having different shapes and angles are described in U.S. Pat. No. 4,149,194, the disclosure of which is incorporated herein by reference in its entirety.
A common problem that arises in digital color halftoning is the manifestation of moiré patterns. Moiré patterns are undesirable interference patterns that occur when two or more color halftone separations are printed over each other. Since color mixing during the printing process is a non-linear process, frequency components other than the fundamental frequencies and harmonics of the individual color halftone separations can occur in the final printout. For example, if an identical halftone screen is used for two color separations, theoretically, there should be no moiré patterns. However, any slight misalignment between the two color halftone separations occurring from an angular difference and/or a scalar difference will result in two slightly different fundamental frequency vectors. Due to nonlinear color mixing the difference in frequency vectors produces a beat frequency which will be visibly evident as a very pronounced moiré interference pattern in the output. To avoid, for example, two-color moiré patterns due to misalignment, or for other reasons, different halftone screens are commonly used for different color separations, where the fundamental frequency vectors of the different halftone screens are separated by relatively large angles. Therefore, the frequency difference between any two fundamental frequencies of the different screens will be large enough so that no visibly objectionable moiré patterns are produced.
In selecting different halftone screens, for example for three color separations, it is desirable to avoid any two-color moiré as well as any three-color moiré. It will be appreciated that in the traditional printing industry that three halftone screens, which can be constructed by halftone cells that are square in shape and identical, can be placed at 15°, 45°, and 75°, respectively, from a point and axis of origin, to provide the classical three-color moiré-free solution.
However, for digital halftoning, the freedom to rotate a halftone screen is limited by the raster structure, which defines the position of each pixel. Since) tan(15° and)tan(75° are irrational numbers, rotating a halftone screen to 15° or 75° cannot be exactly implemented in digital halftoning. To this end, some methods have been proposed to provide approximate instead of exact moiré-free solutions. For example, in U.S. Pat. Nos. 5,323,245 and 5,583,660, this problem is approached by using a combination of two or more perpendicular, unequal frequency screen patterns and non-perpendicular, equal frequency non-conventional screen patterns. However, all these approximate solutions result in some halftone dots having centers that do not lie directly on addressable points, or on the pixel positions defined by the raster structure. Therefore, the shape and center location varies from one halftone dot to another. Consequently, additional interference or moiré between the screen frequencies and the raster frequency can occur. In another approach, U.S. Pat. No. 5,371,612 discloses a moiré prevention method to determine screen angles and sizes that is usable solely for square-shaped, halftone screens.
Customers who use clustered dot halftoning such as laser printing or offset printing may use halftone geometries. However, existing halftone geometries are constrained, capable of providing only limited options with respect to halftone angle and frequency. Given such constraints, it is difficult to satisfy multiple system requirements, e.g. a requirement that halftones be moiré-free, not beat with multiple frequency components from the raster output system, screen visibility, and be free of halftone artifacts. It is also desirable to avoid the use of 0° screens, which give rise to multiple image processing issues. Many attempts have been made to solve these issues, however none have produced a complete solution.
U.S. Pat. No. 7,898,692 to Wang and voce, entitled “Rosette Printing with up to Five Colors” produces moiré-free color halftone printing with up to five color image separations. It also uses a plurality of non-orthogonal halftone screens, defines a first and second color halftone screen fundamental frequency vector for each of three halftone screens which produces moiré-free rosettes, and defines a fourth color halftone with the first fundamental vector of the fourth screen shares a fundamental frequency vector with one of said three halftone screens and a second fundamental frequency vector of the fourth screen shares a fundamental frequency vector with a different one of said three color halftone screens. Further, it defines a fifth color halftone screen where a first fundamental vector of the fifth screen shares a fundamental frequency vector with one of the three halftone screens and a second fundamental frequency vector of the fifth screen shares a fundamental frequency vector with a different one of the three color halftone screens. None of the fundamental frequency vectors of the fifth screen are equal to either of the fundamental frequency vectors of the fourth screen. The disclosure of U.S. Pat. No. 7,898,692 is hereby incorporated by reference in its entirety.
U.S. Pat. No. 7,675,651 to Wang and Lace, entitled “Moiré-free color halftone configuration employing common frequency vectors”, produces moiré-free color halftone printing of up to four color image separations by using a plurality of non-orthogonal halftone screens to produce moiré-free prints that form uniform periodic rosettes. It uses a first and second color halftone screen fundamental frequency vector designed for each of three halftone screens such that the halftone screen set output forms uniform hexagonal rosettes. It also defines a fourth color halftone screen where a first fundamental vector of the fourth screen shares a fundamental frequency vector with one of the three halftone screens. It also defines a second fundamental frequency vector of the fourth screen that shares a fundamental frequency vector with a different one of said three color halftone screens. The disclosure of U.S. Pat. No. 7,675,651 is hereby incorporated by reference in its entirety.
U.S. Pat. No. 7,480,076, to Wang, entitled “Moiré-Free Color Halftone Configuration”, is directed to moiré-free color halftone configurations for clustered dots. Unlike conventional methods, the disclosed method produces periodic hexagon rosettes of identical shapes. These exemplary hexagon rosettes have three fundamental spatial frequencies exactly equal to half of the fundamental frequency of the three halftone screens. The resultant halftone outputs are truly moiré-free, as all the fundamentals and harmonic frequencies are multiples of, and thus higher in frequency than, the rosette fundamental frequency. The disclosure of U.S. Pat. No. 7,480,076 is hereby incorporated by reference in its entirety.
U.S. Pat. No. 6,798,539 to Wang, Fan, and Wen, entitled “Method for Moiré-Free Color Halftoning Using Non-Orthogonal Cluster Screens”, is directed to the use of single-celled, non-orthogonal clustered-dot screens to satisfy the moiré-free conditions for color halftoning. The disclosure also provides methods that combine single-cell non-orthogonal clustered-dot screens and line screens for moiré-free color halftoning. Particularly, the selection of these single-cell halftone screens is determined by satisfying moiré-free conditions provided in the respective spatial or frequency equations. The disclosure of U.S. Pat. No. 6,798,539 is hereby incorporated by reference in its entirety.
U.S. Pat. No. 7,679,787 to Wang and Lace, entitled “N-Color Printing with Hexagonal Rosettes”, produces moiré-free enhanced color halftone printing of color image separations for an arbitrary number of colorants. It uses a plurality of halftone screens to produce outputs that are moiré free and form hexagonal periodic rosettes. A large number of screens can be used for enhanced printing applications, such as printing with high-fidelity colorants, light colorants, or special colorants, such as white, metallics and fluorescents. It defines rosette fundamental frequency vectors VR1, VR2 that satisfy a length and sum requirement to meet visual acceptability standards according to |VR1|>fmin, |VR2|>fmin, and |VR1±VR2|>fmin. It also defines N halftone screens for colorants i=1, N, respectively possessing first and second frequency vectors (Vi1, Vi2), where no two screens possess identical fundamental frequency vector pairs. It then selects fundamental frequency vectors for the N halftone screens according to (Vi1, Vi2)=(mi1VR1+mi2VR2, ni1VR1+ni2VR2) for integer m's and n's, where at least one fundamental frequency vector or its conjugate must also satisfy one of the following: Vik=VR1, Vik=VR2, and “|Vik|>2 max [|VR1|, |VR2|]. The disclosure of U.S. Pat. No. 7,679,787 is hereby incorporated by reference in its entirety. What is needed in the art is a versatile adjustment of a moiré free halftone set such as that angles and frequencies may be optimized for a given imaging printing system.
Incorporation By Reference
S. Wang, Z. Fan and Z. Wen, “Non-Orthogonal Halftone Screens,” Proc. NIP18: International Conference on Digital Printing Technologies, pages 578-584, 2002.
R. Ulichney, “Digital Halftoning,” The MIT Press, pages 117-126, 1988.
M. Turbek, S. Weed, T. Cholewo, B. Damon, M. Lhamon, “Comparison of Hexagonal and Square Dot Centers for EP Halftones,” PICS 2000, pages 321-325.